Field of the Invention
The invention relates to an arrangement for producing electromagnetic radiation, in particular X-ray radiation or extreme ultraviolet radiation, and to a method for operating such an arrangement. In this description, X-ray radiation and extreme ultraviolet radiation are understood to mean electromagnetic radiation which arises as a result of acceleration (deceleration) of particles at a focal spot of a target. The spectral ranges of X-ray radiation and extreme ultraviolet radiation are not restricted by specific wavelengths or photon energies.
The invention also relates, in particular, to the field of examination of industrially and/or technically produced articles by means of electromagnetic radiation. By way of example, it is known to radiate X-ray radiation through a workpiece and to record an X-ray image, to repeat the process for different directions of incidence of the X-ray radiation and then to produce three-dimensional, reconstructed X-ray images of the workpiece by means of a computer. The reconstruction on the computer is referred to as computer tomography (CT).
In particular, a radiation producing device of the arrangement according to the invention can be a microfocus X-ray tube.
In microfocus X-ray tubes, in particular, the size of the focal spot at which impinging particles (usually particles of an electron beam) produce the X-ray radiation is not constant with respect to time. The focal spot generally increases in size as time progresses. In the case of CT, however, high powers of the tubes used for producing the X-rays or other short-wave electromagnetic beams are desirable. As a rule of thumb it holds true that the time for recording the radiographs can be halved if the radiation power of the tube is doubled. As the size of the focal spot increases, however, the spatial resolution in images that could be obtained with an ideally point-type X-ray source is impaired. The temporal change in the size of the focal spot is therefore to be prevented.
U.S. 2001/0050972 A1 describes an X-ray radiation generator that is able to automatically focus an energy beam, e.g. an electron beam. According to the description of the document, there is a close relationship between convergence conditions of an energy beam and the surface temperature of the target of the X-ray tube. Therefore, the document proposes measuring the temperature changes in real time and automatically controlling the current value of a focusing coil.
It appears to be open, however, how meaningful the temperature of the target are for the focusing of the particles (the latter are understood to be in particular electrons or quanta of an electromagnetic radiation). Although the inventor's findings confirm a dependence of the focusing on the temperature of an X-ray tube, the focusing can depend not only on the target temperature but also on other influencing factors, e.g. on how and how effectively the target is cooled.
The method described in U.S. 2001/0050972 A1 additionally presupposes that the optimum focal spot size or the optimum focusing was found in some other way at the beginning of the operation of the X-ray tube. The observation of the change in temperature of the target does not provide any information with regard to this.
Moreover, the optimum focusing can differ in different operating ranges and/or for different operating modes of the radiation producing device.